1. Field of the Invention
The invention relates to hot air dryers, and more particularly to hot air dryers with a plurality of independently controllable drying modules and a plurality of tubes for directing hot air flow.
2. Related Art
Jet tube dryers are known for their ability to direct a large volume of air into intimate and effective contact with web-type products and to handle the return air without inducing uneven secondary drying or temperatures. The intimate contact between primary treating air and the product can be made either gentle or aggressive by adjusting air velocity and other variables. The overall efficiency of any drying operation depends on this intimate contact over a large area and the resulting fast transfer of moisture or heat between the product and the treating air.
FIG. 6 shows the various components found in a double impingement jet-tube dryer. This design includes an oil or gas direct fired unit or a steam or hot oil coil indirect fired unit. The circulating air blower provides a positive pressure in the plenum, which forces air through the tubes at a high velocity. Between the blower and the plenum that distributes the air evenly to the jet tubes is a damper used to control the circulating air volume and the air velocity in the jet tubes. The entrance to each jet tube is flared to reduce turbulence and entrance losses. Air velocities of several thousand feet per minute are common.
The length of the jet tubes is determined by the space necessary to carry the return air through the forest of tubes at a nominal velocity. This low velocity keeps the return air from competing with the primary air from the jets and causing uneven secondary drying. In a drying operation, air in the plenum and in the tubes is hotter than the return air, and, therefore, no moisture carried by the return air will condense on the plenum or tube surfaces. The product conveyor can be a series of carrying rolls, a tenter, fixed bars, or a belt.
After the circulating air leaves the tube forest, and before it is reheated, the amount of air necessary to carry away the moisture being removed from the product is exhausted. Circulating air is generally filtered at a point between the exhaust and make-up air ports. This is because the circulating air is coolest and at its least volume. The filtering process employs either continuous self-cleaning or quickly replaceable screens.
Sufficient make-up air is usually brought into the dryer just after the filter screen. This location overcomes any negative pressure in the housing caused by exhausted air and also allows the cool air to be heated and well mixed before contacting the product. When heating with oil, this excess fresh air also assists in obtaining complete combustion.
Today, gas/oil combination burners are standard and cost no more than single fired units. Even though only one fuel is used initially, a second fuel can be added at a minimum expense, when needed. A direct fired burner is shown in FIG. 6. These are efficient and usually are acceptable even though the products of combustion are circulated in the air. Indirect burners pass the products of combustion through an air-to-air heat exchanger. Not only are these installations more expensive, but they are less efficient.
The air can be heated by a steam coil or hot oil coil. Steam coil installations are less expensive than direct fired gas installations, while oil coil installations cost somewhat more. The temperature of the heated air is sensed by an air temperature sensing bulb, which controls the burner or coil output and is located in the pressure plenum. Not shown in FIG. 6 are the access doors required for cleaning, maintenance and explosion relief (when necessary). Many dryers are similar to FIG. 6, except that they also have plenums and jets under the product to treat both sides simultaneously.
FIG. 7 shows a tube forest in a double impingement jet tube dryer, which is effective for drying tubular knit products. The material is carried by a flat wire conveyor having an open mesh, which allows the air to penetrate the belt with minimum interference. By increasing the air velocity on the bottom jets, the material can be lifted off the belt to allow unrestricted shrinkage. On suction drum dryers, on the other hand, the product is held tightly to the drum, restricting its ability to shrink. Such is also true on drum dryers where high-velocity air on the outside holds the product on the drum. When only part of the suction drum surface is covered by material, air short circuits through the uncovered areas and reduces drying.
Conversely, the double impingement jet tube dryer shown in FIG. 7 treats the product uniformly, regardless of how much conveyor surface is uncovered. Also, the return air travels in a path through the forest of tubes and is prevented from interfering with the treating air jet stream.
FIG. 8 shows a dryer with air jets located above a closely woven steel belt and with a suction plenum located below. This design uses the "flow-through" principle. The belt creates enough resistance to discourage excessive short-circuiting of the treating air when the belt is only partially covered by the product. With this design, suction is not required to hold the material against the belt for transport, as with a drum dryer. For maximum efficiency, adjustable slides can be supplied to vary the belt open mesh width in direct proportion to a variable web width product mix. The tubular air jets create a uniform application of air on the belt. Any excess treating air returns through the tube forest as in the standard design.
The FIG. 8 design is advantageous for fragile, porous products such as nonwovens, laces, etc., especially if a flat, starched-like final product is desired.
FIG. 9 shows a standard pattern used to assure uniformity of treatment with jet tube dryers. Section A indicates the work area under each air jet tube, approximately 80% of the total treatment area, section B indicates the inactive area under each air jet tube, and section C indicates the inactive collision area between air jets.
If the product is moving under the jet tubes in the direction of the arrows in FIG. 9, it receives uniform treatment after passing under four rows of tubes. If passed under only one row of tubes, stripes of dry and wet product are created. If the product is stopped beneath the jets, the illustrated white areas receive high velocity drying treatment while the shaded areas do not.
A large proportion (80%+) of the total product area receives high velocity air drying treatment at any instant. This feature, combined with the intimate and efficient contact between air and product, accounts for the fast treatment and high overall efficiency of jet tube dryers. The tubes are arranged on an angle to eliminate streaking and provide a uniform distribution of the treating air.
FIG. 10 is a cross section of the product and jet tube section taken on line 10--10 of FIG. 9. Depicted are the relative air velocities at various points in the air projection system. Air emitted from the positive supply plenum through the jet tubes moves at a much higher velocity than the return air. The laminar air flow from the tubes bounces off the product and returns upward into the less positive area within the forest of tubes. For this reason, the return air does not interfere with the high velocity treatment air, nor does it contact the product. This eliminates overdrying of the webs' salvages caused by return air and short-circuiting treatment air, which plagues most other types of dryers. Uniformity across the web can be accomplished with the jet tube dryer, regardless of the web width. On narrow dryers, return air is usually taken to one side; but for wide machines, it is taken to both sides.
Dryers are notorious for being major energy consumers as well as being extremely inefficient, that is, the energy used, as compared to the work actually performed, is quite high. The basic energy losses are: